Support and resistance is one of the most widely used concepts
in forex trading. When the forex market moves up and then pulls
back, the highest point reached before it pulled back is now
resistance.
Ref: alpari.com/en/analytics/reviews/
As the market continues up again, the lowest point reached
before it started back is now support. In this way, resistance and
support are continually formed as the forex market oscillates over
time. The reverse is true for the downtrend.